Genomic imprinting refers to the process wherein the maternal and paternal contributions to the embryonic genome are programmed to function differently. The existence of genetic imprinting has, in part, been proven by the performance of nuclear transplantation in the early mouse embryo. Recent evidence suggests that imprinting may be a more general phenomena than originally believed and several instances in which imprinting has been implicated as a cause of human disease now exist. These examples include the parentally modified onset of such diseases as Huntington's disease, myotonic dystrophy, and spinocerebellar ataxia. In addition syndromic diagnoses such as Prader-Willi and Angelman syndrome are now felt to represent examples of human genomic imprinting. The current work attempts to define when in development imprinting occurs. The proposed experiments involve using male and female germ cell lineage nuclei in nuclear transfer experiments to determine if they possess an imprint consistent with the sex of the donor from which they were obtained. This analysis will begin with primordial germ cell nuclear donors obtained from 1417 day old mouse fetuses and extend to later stages of gametogenesis if necessary. Nuclear transfer embryos will be cultured both in vivo and in vitro and analyzed for previously described androgenetic and gynogenetic phenotypes.In addition the ability of ovum cytoplasm to modify and potentially reverse the imprint at the level of the entire genome or at the level of a single imprinted locus, fused will be determined. It is hoped that by knowing when in development imprinting occurs future experiments that deal more directly with the molecular mechanism of imprinting will be facilitated. The proposed experiments will also address whether nuclear reprogramming can be facilitated in nuclear transplant embryos. The results of these experiments could have considerable impact on the feasibility of cloning in domestic animals.